Fuel cells are electrochemical devices that convert chemical potential energy into usable electricity and heat without combustion as an intermediate step. Fuel cells are similar to batteries in that both produce a DC current by using an electrochemical process. Two electrodes, an anode and a cathode, are separated by an electrolyte. Like batteries, fuel cells are combined into groups, called stacks, to obtain a usable voltage and power output. Unlike batteries, however, fuel cells do not release energy stored in the cell, running down when battery energy is gone. Instead, they convert the energy typically in a hydrogen-rich fuel directly into electricity and operate as long as they are supplied with fuel and oxidant. Fuel cells emit almost none of the sulfur and nitrogen compounds released by conventional combustion of gasoline or diesel fuel, and can utilize a wide variety of fuels including natural gas, coal-derived gas, landfill gas, biogas, alcohols, gasoline, and diesel fuel oil.
Several types of fuel cells are under development. Among these, the solid oxide fuel cell (SOFC) is regarded as the most efficient and versatile power generation system, particularly for dispersed power generation, providing low pollution, high efficiency, high power density and fuel flexibility. In transportation applications, SOFC power generation systems are expected to provide a higher level of efficiency than conventional power generators which employ heat engines such as gas turbines and diesel engines that are subject to Carnot cycle efficiency limits. Therefore, use of SOFC systems as power generators in vehicle applications is expected to contribute to efficient utilization of resources and to a relative decrease in the level of CO2 emissions and an extremely low level of NOx emissions. SOFC systems designed to address specific concerns and requirements of operation in a vehicle are under development including SOFC systems designed to serve as an auxiliary on-board power unit rather than as the prime energy source of the vehicle.
As with fuel cells generally, very hot solid oxide fuel cells (SOFC) having high electrical conductivity, are used to convert chemical potential energy in reactant gases into electrical energy. In the SOFC, two porous electrodes (anode and cathode) are bonded to an oxide ceramic electrolyte (typically, yttria stabilized zirconia, ZrO2—Y2O3) disposed between them to form a selectively ionic permeable barrier. Molecular reactants cannot pass through the barrier, but oxygen ions (O2−) diffuse through the solid oxide lattice. The electrodes are typically formed of electrically conductive metallic or semiconducting ceramic powders, plates or sheets that are porous to fuel and oxygen molecules. Manifolds are employed to supply fuel (typically hydrogen, carbon monoxide, or simple hydrocarbon) to the anode and oxygen-containing gas to the cathode. The fuel at the anode catalyst/electrolyte interface forms cations that react with oxygen ions diffusing through the solid oxide electrolyte to the anode. The oxygen-containing gas (typically air) supplied to the cathode layer converts oxygen molecules into oxygen ions at the cathode/electrolyte interface. The oxygen ions formed at the cathode diffuse, combining with the cations to generate a usable electric current and a reaction product that must be removed from the cell (i.e., fuel cell waste stream) or recycled such as with a waste energy recovery device.
Individual fuel cells are stacked anode to cathode, to provide a fuel cell stack providing the desired output voltage. Conductive, typically metal, plates referred to as interconnects are interleaved between each fuel cell, as well as at each end of a fuel cell stack and at each side of a single fuel cell. One function of the interconnect is to convey electrical current away from the fuel cell and/or between adjacent fuel cells and heat away from the fuel cell or cells. The interconnect, therefore, should have a relatively high electrical conductivity to minimize voltage losses, with negligible contact resistance at the interconnect-electrode interface. The interconnect should further have a relatively high thermal conductivity to provide uniformity of heat distribution and to reduce thermal stresses. A thermal conductivity above about 25 W/m K, for example, is desirable. Since an intermediate interconnect in a fuel cell stack extends between the anode of one fuel cell and the cathode of the adjacent fuel cell, the interconnect must be impervious to gases in order to avoid mixing of the fuel and oxidant. Thus, the interconnect should have a relatively high density with no open porosity, as well as stability in both oxidizing and reducing environments at the operating temperature. The interconnect should further have high creep resistance so that there is negligible creep at the operating temperature and a low vapor pressure. The interconnect should further exhibit phase stability during thermal cycling, have a low thermal expansion mismatch between cell components, and have chemical stability with respect to components with which it is in contact. The interconnect should possess sufficient strength to provide structural support to the relatively fragile fuel cells. In addition, the interconnect should preferably be low cost, easily fabricated, and have low brittleness.
A second function of the interconnect is to provide gas flow passages on the top and bottom surfaces while maintaining good electrical contact to the fuel cell. The gas flow passages are preferably configured to minimize flow pressure drop of the gas streams while promoting swirl or mixing for good fuel utilization (anode) and heat transfer (cathode). The gas flow passages are connected to supply and return manifolds which can be discrete devices or integral to the interconnects and fuel cells.
Ceramic, cermet and alloy interconnects are known in the art. Metallic materials have the advantages generally of high electrical and thermal conductivity and ease of fabrication. However, stability in a fuel cell environment comprising high temperatures in both reducing and oxidizing atmospheres, limits the number of available metals that can be used in interconnects. Most high temperature oxidation resistant alloys have some kind of built-in protection mechanism, typically forming oxidation resistant surface layers. Fabrication of such interconnects is complex and may comprise, for example, providing three sheet metal sheets having appropriate gas flow channels formed therein and combining the sheets, such as by brazing, to form an interconnect assembly having anode gas (fuel) channels on one side, cathode gas (oxidant, typically air) channels on the opposite side, and integral anode gas supply channels. A support for the ceramic fuel cell is provided on the anode side, such as a sheet of nickel foam, which foam support provides flow passages for the anode gas and electrical connection from the cell to the interconnect. The anode gas supply channels must be sealed from one interconnect to the next, such as with a non-conducting gasket.
What is needed in the art is a simplified, lower cost, high efficiency interconnect for fuel cell elements.